Focus on the intrinsic
We've all heard of the carrot and stick approaches to motivation. Our best case scenario as instructional designers is that our learners are learning something because they truly love it and just want to know more. More often than not, however, learners are told they have to complete a set of training, or they're guilted into it. These are motivational approaches, albeit, not the ones we want to encourage.
Help your learners get as close to intrinsic as possible. One place to start is to fulfill their three psychological needs.
Here are a few other resources as well:
Understanding intrinsic vs extrinsic motivation in the classroom